For a substrate in which at least a portion adjacent to a surface is made of a refractory material containing silicon, the invention relates to protecting the substrate while it is being used at high temperature in a medium that is oxidizing and wet, by forming on the surface of the substrate an environmental barrier that does not contain boron.
The invention seeks in particular to protect refractory materials constituted by monolithic ceramics, e.g. silicon carbide SiC or silicon nitride Si3N4, and more particularly to protect refractory composite materials such as ceramic matrix composite (CMC) materials containing silicon, e.g. CMCs having a matrix that is constituted at least in part by SiC.
A particular field of application of the invention is protecting CMC parts that form hot parts of gas turbines, such as combustion chamber walls, or turbine rings, turbine nozzles, or turbine blades, for aeroengines or for industrial turbines.
For such gas turbines, the desire to improve efficiency and also to reduce polluting emissions is leading to the use of ever-higher temperatures in combustion chambers.
Proposals have thus been made to replace metal materials by CMC materials, in particular for the walls of combustion chambers or turbine rings. CMC materials are known to possess simultaneously both good mechanical properties enabling them to be used as structural elements, and the ability to conserve these properties at high temperatures. CMC materials comprise fiber reinforcement made of refractory fibers, typically carbon fibers or ceramic fibers, and densified by a ceramic matrix, e.g. a matrix of SiC.
In a corrosive environment (oxidizing atmosphere, in particular in the presence of moisture and/or a saline atmosphere), a phenomenon of surface retreat is observed with CMC materials having an SiC matrix because of volatilization of the silica (SiO2) that is formed by oxidation at the surface of the CMC material.
Recommendations have been made to form an environmental barrier coating (EBC) on the surface of the CMC material. In one such known barrier for a substrate of CMC material having an SiC matrix, the anti-corrosion function is provided by a layer made of an aluminosilicate type compound of an alkaline earth metal, such as the compound BaO0.75.SrO0.25.Al2O3(SiO2)2 commonly referred by the abbreviation BSAS. A chemical barrier layer made of mullite or a mixed layer comprising a mixture of BSAS and mullite is interposed between the substrate and the anti-corrosion layer in order to avoid chemical interactions between the BSAS of the anti-corrosion layer and the silica that is formed by oxidation of the final SiC layer of the substrate. A silicon layer is formed on the substrate to enable the chemical barrier layer to bond thereto. Such an environmental barrier is described in particular in documents U.S. Pat. No. 6,866,897 and U.S. Pat. No. 6,787,195. The various layers are typically formed by physical deposition, in particular by thermal plasma deposition.
Satisfactory behavior for that environmental barrier has been observed at temperatures of up to 1200° C. approximately, but significant degradation is observed when the temperature exceeds 1300° C. Indeed, the bonding layer made of silicon or the surface of the matrix of the substrate material containing silicon is easily oxidized into silica in the event of cracks or defects, such as flakes, being created in the environmental barrier while the material is in use. At temperatures of about 1310° C. and above, this formation of silica leads to a chemical interaction between the BSAS of the mixed BSAS+mullite layer and/or of the final layer and the silica that is formed by oxidation, thereby leading to the environmental barrier being degraded more or less quickly and completely depending on the size of the defect therein.
Proposals are also made in an article by F. Smeacetto et al. (“Protective coatings for carbon-bonded carbon-fiber composites”, Ceramics International 34 (2008), pp. 1297-1301) for a method of protecting carbon/carbon composite that consists in forming an SiC surface layer by chemical reaction with molten silicon and then depositing a silica-based glass coating containing yttrium oxide and alumina. The glass is prepared by mixing its constituent oxides while molten. The coating is formed by surface deposition of a slip containing the powder of the prepared glass, followed by heat treatment. Microcracks in the coating can be healed by raising the temperature to 1375° C. to cause the surface to be coated with molten glass. The indicated maximum operating temperature is 1400° C. Nevertheless, it should be observed that the melting of the coating at 1375° C. makes it difficult to envisage use above that temperature, in particular when the surface is exposed to a stream of gas at high speed, since it is then possible for the coating to be blown away.
Document U.S. Pat. No. 6,759,151 discloses a protective coating, in particular for a ceramic containing silicon, the coating comprising a bonding layer, at least one intermediate layer, and an outer layer. The outer layer is a rare earth silicate or is based on hafnium oxide or on zirconia.
Document US 2003/0138641 also relates to a protective coating for a ceramic containing silicon, the coating being made of stabilized zirconia with an optional intermediate layer made of rare earth silicate.
Document US 2006/0073361 describes a protective coating having an outer layer of stabilized zirconia and a stack of functional intermediate layers possibly containing rare earth silicates.